Servocontrols in theater and mechanical-creature applications

Putting motion into art requires a different approach to motion control than typical machine-tool applications. Although hardware elements such as actuators, servomotors, and controllers are similar, the approach to motion in artistic endeavors is typically quite different. In contrast to industrial applications, artistic motions are often only roughly defined while the art piece or lifelike creature is being designed. The artist then experiments to make the motions look natural and bring them to “life.”

Industrial applications are usually characterized by motion tasks that are precisely repetitive and optimized for throughput. But some mechanized art is pure fantasy, with motions based solely on artistic whim. An art piece’s movements begin with an engineer or programmer commissioning the control loops and homing routines, and verifying the available motion dynamics. The various artwork shown in the accompanying images was commissioned using Window-based software called QuickControl. It is used to test and embed the control parameters, start-up and homing routines, and communications parameters into nonvolatile memory embedded into the controllers.

The development of the artistic motion methods depends on where and how the art will be used. Amusement parks and museums use hardware that will “carry the show” without much human interaction. In contrast, the motion-film industry depends on the real-time interpretation of “puppet masters” interacting with the actors to meet the artistic vision of the director. These different approaches require different motion-system architectures and communications protocols.

Amusement-park designs use a specialized stage controller to store and play both the sounds that will be used and the motion and lighting information in coordinated data sets called tracks. The stage controllers are usually configured through a laptop by the engineer/artist to match the motions to the desired timing. Overall motion design is often guided by sound tracks which provide the show tempo.

The basic motions are first selected from the primitives supplied — sine waves, ramps, parabolic sections. These are literally drawn into a tract on screen, stretching the amplitude and duration. More precise motions can then be sculpted from the initial smooth curve primitives by stretching and warping sections of them on screen into more-complex trajectories.

Developers view the results by playing the motion tracks to the hardware along with the sound tracks. The structures being moved are often flexible — trunks, tails, fins — so motion must be tailored to prevent shocks that will make them wiggle — unless that is the desired effect. Developers iterate the motion editing until they create the final fluid motions.

A simple figure may only have four or five actuators, while a more complex figure might include 15 or more. The stage controller stores tracks for each of these axes for multiple figures, as well as the lighting, spot lights, strobe lights, smoke generators, and other special effects.

Motion controllers, dimmer racks, and other stage equipment play back motion and effects tracks using a serial protocol called DMX-512A (or ANSI E1.11-2008). Under the hood, DMX uses a 250-Kbaud serial communications over an RS-485 network. The data is sent in bursts of data, called frames. The data for each device is typically updated at 30 to 60times/sec — 44 Hz is the maximum rate with a full 512-byte data payload and optimal timing, with faster updates for a smaller data payload. Each frame starts with a break (extended-start bit state) followed by mark level and a start byte (to identify the frame). Then comes up to 512 data bytes, each called a slot. Each 8-bit slot can carry 256 levels of information which is usually adequate to control lighting and to provide simple motions.

Fluid motion, though, requires the use of advanced controllers, such as those from Gilderfluke & Co., Burbank, Calif. They can combine and manipulate multiple slot data as 16, 24, or 32-bit chunks for greatly improved dynamics.

Motion controllers such the SilverDust and SilverSterling types from QuickSilver Controls Inc., Covina, Calif., can interpret this multiple slot data, allowing more fluid motion. These controllers can also collect multiple parameters from the DMX data stream. Each parameter may be configured for 8, 16, 24, or 32-bit data. The data can be sent in signed or unsigned format, and as Big Endian (most-significant byte first) or Little Endian (least-significant byte first). This capability provides great flexibility of motion for each axis.

The DMX data stream updates each parameter nominally 44 times/sec, with the actual rate varying according to the number of devices being controlled and how many slots of data each device requires. Each individually controlled light usually requires one slot to provide 256 light levels. High-power LED lighting with red, green, blue, and white light-emitting diode sets consumes four slots. A precise motion actuator typically uses four to 12 slots. A smoke machine may use two slots — one for fan speed and the other to control the quantity of smoke. If fewer than 512 slots are needed, the frame is shorter and may be sent in less time.

The data stream update rate also varies depending on whether multiple styles of frames — called universes — go over the same serial link. The default universe uses a start byte with a value of 0. Other universes with different start bytes can be used to deliver additional banks of up to 512slots each. Certain reserved universe codes may be used to retrieve diagnostic or configuration information (from devices that support bidirectional data). Alternate start codes are standardized and coordinated by ESTA to help guarantee that equipment from different manufacturers will work together.

Data corruption should be rare in properly configured systems, but statistics say it is hard to avoid completely. The DMX data stream does not support a retry of corrupted frames. It just continues on with the next frame, so some DMX networks employ error detection. A checksum is one such technique where both the stage controller and the error-checking controllers add the data values from, say, slot 0 through 100, to form the checksum. The stage controller sends the computed checksum information in the next two slots; the motion controller compares the received checksum with its own calculation. When these calculations do not agree, the controller discards the errant data and waits for the next frame.

Because DMX does not resend data following error detection, controllers must estimate the missing data from what they have. The motor controller filters and interpolates data points across sample periods to produce smooth motion.

QuickSilver controllers separate the control constants associated with feed-forward from those used for feedback. Reducing the feed-forward-control constants compared with their corresponding feedback-control constants effectively filters the motion, causing a smooth exponential decay through the motion transitions. Electronic-damping techniques can be used to add simulated viscous inertial damping, further smoothing motions. This intentional “detuning” of the control loop results in filtered motions which mimic nature much more closely than the crisp motions of the tightly tuned control loops typically found in the machine world. Trapezoidal motions that come to a sudden and complete stop with no overshoot or undershoot or ringing are highly prized in machinery but are uncommon in nature.

Additional channels in DMX can also be used to adjust the control loop dynamically, tightening time sequences that need rapid motion and then loosening them by reducing gains to again produce smooth gliding moves. Quick motions such as an eye blinking or rapid tracking need controllers operating at higher gains. The glittering of a replica’s eye is actually a series of quick motions with pauses in between. Slower and smoother motion for the sliding glance of an eye might use less gain to generate moves that are less crisp.

The film industry’s live-action requirements necessitate different approaches to motion control. Rather than capturing and playing motions again and again, art pieces for film are controlled in real time by skilled puppeteers. Capturing a scene often demands multiple takes with slight variations introduced each time. The capability to interact with the actors in real time makes the scene more natural. It also prevents long downtimes for the crew and actors while a motion is being reprogrammed. Fast retakes can also help prevent distractions related to moving shadows or changing weather when shooting on scene.

Puppeteers commonly use radio-control (R/C) consoles to control the action of robotic characters. Advanced R/C controllers are highly programmable. They can blend multiple sources and can limit rate of change as well as the maximum excursions. These consoles multiplex the multiple data sources in time to produce the transmitted signal. A single R/C controller can typically handle from four to as many as 15 actuators. At the receiver, the signal is commonly demultiplexed to three pin connectors carrying power (~5V), ground, and pulse-width modulation (PWM) for each axis.

Standard R/C controllers and R/C servomotors nominally use a 1 to 2-msec pulse width modulation signal, 0 to 3.3 V, to define the needed position or speed. Some units can use extended 0.7 to 2.3-msec timing. In the case of nominal timing, a 1-msec pulse indicates extreme motion position or velocity in one direction; a 1.5-msec pulse represents centered or stopped; and a 2-msec pulse indicates the opposite extreme position or direction of motion.

The receiver provides the PWM signal to each axis, updating each approximately every 20 to 30 msec. More-sophisticated servos retain the latest commanded input value in case of a signal loss, to prevent a change until the signal is reacquired. Smaller axes can be handled by the conventional small R/C servos typically seen in model airplanes. Larger-power axes use the PWM input command capability of higher-power controllers. SilverDust controllers recently introduced PWM capture capability, allowing direct R/C control. This technology is currently animating a robotic animal figure, which contains a dozen 200-lb actuators, in a movie now being filmed.